I want to configure few pens, e.g. a blue of this size, a red of that size, a black.
Then I want to be able to simply press 1 or 2 or 3 to toggle between them.

I know the top voted "idea" is "fewer clicks to change a tool", my version is a lot more specific:
- Zero clicks
- A short list of pre-configured (by the user) pens
- Keyboard shortcut to switch between them